1. Field of the Invention
The present invention relates to a sub-mount type device for emitting light and, more particularly, improvement in speed of its response.
2. Description of the Prior Art
FIG. 1 shows in perspective a light amplification by stimulated emission of radiation (LASER) chip 2 obtained by subjecting a P type substrate to epitaxial growth technique. Referring to FIG. 1, on the upper face of the P type substrate 5, a current-blocking layer 7, a cladding layer 9, a P type activation layer 11, a cladding layer 13 and an N type cap layer 15 are applied in this order. In this type of LASER chip 2, the activation layer 11 can emit light.
Each layer on the substrate 5 is very thin compared with thickness of the substrate 5. Actually, the thickness of the substrate 5 is a couple of ten of .mu.m while the total thickness of the current-blocking layer 7, the cladding layer 9, the activation layer 11, the cladding layer 13 and the cap layer 15 is a couple of .mu.m. Therefore, the activation layer is nearer to a chip negative electrode 20 applied on the cap layer 15 than to a chip positive electrode 18 applied on the lower face of the substrate 5.
A heat sink is Generally provided for the LASER chip 2 which Generates heat from the activation layer 11. Specifically, as shown in FIG.2, the LASER chip 2 is fixed to a heat sink 24 of large heat capacity so that the chip negative electrode 20 of the chip 2 attaches to the heat sink 24. That is, the LASER chip 2 of FIG. 1 is overturn and fixed to,the heat sink 24. That is because the activation layer 11 where heat is Generated is near to the chip negative electrode 20 than to the chip positive electrode 18.
Note that it is desirable that electric potential of the heat sink 24 is zero in order to prevent the electrical trouble because the heat sink 24 is connected with a case (not shown). Therefore, when power supply of positive is used the heat sink 24 is in negative side and when power supply of negative is used the heat sink 24 is in positive side. Since in semiconductor LASER there is many cases where power supply of positive is used it must be arranged that the heat sink 24 is at ground. That is why the chip negative electrode 20 is connected with the heat sink 24 as shown in FIG. 2.
As is described above, the positioning of FIG. 2 permits the heat sink 24 to be at Ground and to be brought out its capability of heat radiation.
Meanwhile, nowadays, a LASER chip is obtained by subjecting a N type substrate to metal organic chemical vapor deposition (MOCVD) technique or molecular beam epitaxicy (MBE) technique. FIG. 3 shows a LASER chip 30 obtained in any one of these ways. Referring to FIG. 3, differently from a LASER chip using P type substrate, an activation layer 10 formed on an N type substrate 81 is near to a chip positive electrode 32 than to a chip negative electrode 34. Therefore, when the LASER chip 30 is fixed to the heat sink 24 of large heat capacity so that the chip negative electrode 34 attaches to the heat sink 24 its capability of heat radiation may be brought out because the activation layer 10 where heat is generated is away the heat sink 24. On the other hand, when the LASER chip 30 is fixed to the heat sink 24 of large heat capacity so that the chip positive electrode 32 attaches to the heat sink 24 such a problem disappears but occurs a new problem where the heat sink 24 is not relatively at negative.
To solve the problem, a sub-mount body 26 is provided between the LASER chip 30 and the heat sink 4 as shown in FIG. 4. The sub-mount body 26 is constructed using a conductive substrate 36 with silicon, a insulating layer 38 of silicon oxide, a connecting electrode 40, a positive electrode 42 and a negative electrode 44, wherein the insulating layer 38 is formed on the upper face of the substrate 36, the positive electrode 42 attaches to the upper face of the insulating layer 38, the negative electrode 44 attaches to the upper face of the substrate 36 and the connecting electrode 40 attaches to the lower face of the substrate 36, the sub-mount body 26 is connected with a heat sink 4 through the connecting electrode 40. The LASER chip 30 is connected with the sub-mount body 26 wherein the chip positive electrode 32 attaches to the positive electrode 34 of the LASER chip 30 and the negative electrode of the sub-mount body 26.
The heat sink 4 is connected with the chip negative electrode 34 through the connecting electrode 40, the substrate 36, the negative electrode 44 and the golden wire 46. The fact permits the heat sink 4 to be at ground. Further, it is arranged that the activation layer 10 where heat is generated is positioned near to the sub-mount body 26. The fact permits the generated heat to be radiated sufficiently.
As is described above, in the case of the LASER chip 30, formed by using the N type substrate the sub-mount body 26 is provided between the heat sink 4 and the chip 30, which can solve the problem with heat radiation and electric potential.
Undesirable electrostatic capacity, however, is generated between the:positive electrode 42 and the conductive substrate 36 because the sub-mount body 26 have a capacitor structure wherein the insulating layer 38 is sandwiched between the positive electrode 42 and the conductive substrate 36. In view of the equivalent circuit 44 shown in FIG. 5, an electrostatic capacity C is parallel connected to the element of the chip 30. Therefore, voltage change in the element of the chip 30 can not occur rapidly according to voltage change between a terminal 50 and a terminal 52, which bar high speed response of the LASER chip 30. Since such a delay of response increases in proportion to volume of the electrostatic capacity, in order to obtain high speed response it is desirable that the electrostatic capacity can be reduced possibly.
Reduction in electrostatic capacity can be effected by thickening the insulating layer 38. However, when the insulating layer 38 is thick the generated heat can not radiate from the chip sufficiently because of increase in thermal resistance.